Remove some unused Classes

These classes were deleted because they have no references in our code base. They are not in Spring execution flow nor instantiated with "new":
- com.cloud.agent.api.CheckStateAnswer
- com.cloud.agent.api.StartupVMMAgentCommand
- com.cloud.agent.api.routing.UserDataCommand
	- remove from description at
com.cloud.configuration.Config.ExecuteInSequenceNetworkElementCommands
enum
- com.cloud.agent.api.storage.UpgradeDiskCommand
- com.cloud.agent.api.storage.CreatePrivateTemplateCommand
- com.cloud.agent.api.storage.DestroyAnswer
	- Note: "FIXME: Should have an DestroyAnswer" at
com.cloud.storage.resource.StoragePoolResource
- com.cloud.agent.api.storage.UpgradeDiskAnswer
- com.cloud.agent.api.storage.ManageVolumeAvailabilityAnswer
- com.cloud.agent.api.storage.ManageVolumeAvailabilityCommand
- com.cloud.exception.UsageServerException
- com.cloud.info.SecStorageVmLoadInfo
- com.cloud.serializer.SerializerHelper

Author: rafaelweingartner